With the advancement of ubiquitous network society, there is a large demand for mobile devices such as cellular phones, notebook personal computers, and digital still cameras. As the power source for mobile devices, it is desired to put fuel cells, which do not have to be recharged and can continuously supply power to devices if get refueled, into practical use as early as possible.
Among fuel cells, direct oxidation fuel cells, which generate power by directly supplying an organic fuel such as methanol or dimethyl ether to an anode for oxidation without reforming it into hydrogen, are actively studied and developed. Direct oxidation fuel cells are receiving attention in terms of the high theoretical energy densities of organic fuels, system simplification, ease of fuel storage, etc.
A direct oxidation fuel cell has a unit cell composed of a membrane-electrode assembly (MEA) sandwiched between separators. The MEA is composed of a solid polymer electrolyte membrane sandwiched between an anode and a cathode, and each of the anode and the cathode includes a catalyst layer and a diffusion layer. Such a direct oxidation fuel cell generates power by supplying a fuel and water to the anode and supplying an oxidant to the cathode.
For example, the electrode reactions of a direct methanol fuel cell (hereinafter referred to as a “DMFC”), which uses methanol as the fuel, are as follows.Anode: CH3OH+H2O→CO2+6H++6e−Cathode: 3/2O2+6H++6e−→3H2O
On the anode, methanol reacts with water to produce carbon dioxide, protons, and electrons. The protons produced on the anode migrate to the cathode through the electrolyte membrane, and the electrons migrate to the cathode through an external circuit. On the cathode, these protons and electrons combine with oxygen to form water.
However, practical utilization of DMFCs has some problems.
The anode of DMFCs has a low catalytic activity (specific activity) to oxidize methanol. Hence, the anode overvoltage of DMFCs is markedly high compared with that of solid polymer electrolyte fuel cells (hereinafter referred to as PEFCs) which use hydrogen as the fuel. Further, due to the so-called “methanol crossover”, i.e., permeation of unreacted methanol through the electrolyte membrane to the cathode, methanol oxidation reaction occurs on the cathode in addition to oxygen reduction reaction (cathode electrode reaction). The methanol oxidation reaction causes an increase in cathode overvoltage. For these two reasons, the overvoltage of DMFCs is increased, so that the power density of DMFCs is lower than that of PEFCs.
To address these problems, there has been proposed a method of increasing the amount of catalysts included in a DMFC relative to a PEFC to increase the surface area of the catalysts per unit electrode area. However, an increase in the amount of a catalyst leads to an increase in the thickness of the catalyst layer itself, so that it becomes difficult for methanol or air (oxygen) to reach the reaction site inside the catalyst layer. As a result, the power generating characteristics degrade. When the pore size of the catalyst layer is enlarged to avoid such a problem, the electronic conductivity and proton conductivity lower significantly.
Hence, in order to solve the above-discussed problems, many proposals have been made to improve the structure of the catalyst layers themselves.
For example, Japanese Laid-Open Patent Publication No. 2005-183368 (Document 1) discloses that each of the anode catalyst layer and the cathode catalyst layer has a thickness of 20 μm or more, that at least one of the catalyst layers has pores with pore sizes of 0.3 to 2.0 μm, and that the volume of these pores is equal to or greater than 4% of the volume of all the pores. With such configuration, Document 1 intends to facilitate the supply of liquid fuel and air (oxygen) to respective reaction sites inside the electrodes without lowering electronic conductivity and proton conductivity.
Japanese Laid-Open Patent Publication No. 2005-197195 (Document 2), which is not directed to a DMFC, discloses that at least one of the catalyst layers has a laminate structure composed of a layer with pores for promoting gas supply (“porous layer”) and a layer without such pores (“non-porous layer”), and that the porous layer is disposed on the gas diffusion layer side. With such configuration, Document 2 intends to efficiently supply reactant gas to the catalyst contained in the catalyst layer and suppress an increase in the electron transfer resistance of the catalyst layer.
Japanese Laid-Open Patent Publication No. Hei 8-162123 (Document 3), which is not directed to a DMFC either, discloses that the size of clusters of catalyst particles and ion-exchange resin is small on the polymer electrolyte membrane side and large on the current collector side. With such configuration, Document 3 intends to facilitate the supply of reactant gas and the removal of generated gas and increase the utilization rate of the catalyst.
However, according to such conventional configurations, it is difficult to obtain a catalyst layer with a small overvoltage in which electronic conductivity and proton conductivity are ensured and the diffusion of fuel or air and the removal of carbon dioxide or water (reaction product) are improved.
In the case of the technique represented by Document 1, the lower limit value of thickness of the catalyst layer, the pore size, and the pore volume are merely specified, and it is difficult to say that the whole catalyst layer has an optimum pore structure that is excellent in all of the diffusion of fuel or air, the removal of carbon dioxide or water (reaction product), electronic conductivity, and proton conductivity.
In the case of the technique represented by Document 2, the catalyst layer has a laminate structure of two or more layers; the porous layer on the gas diffusion layer side is provided with the function of diffusing fuel or air and removing carbon dioxide or water produced, while the non-porous layer on the polymer electrolyte membrane side is provided with the function of electron conductivity. However, since actual electrode reaction takes place in the three-phase interface of catalyst/electrolyte/reactant gas (fuel or air), this configuration may have the problem of degradation of power generating characteristics due to insufficient distribution paths of reactant gas in the non-porous layer on the polymer electrolyte membrane side.
In the case of the technique represented by Document 3, the catalyst layer near the polymer electrolyte membrane is characterized as the main site of electrode reaction, and the catalyst layer near the current collector is characterized as the site where the supply of reactant gas and the removal of generated gas are facilitated. However, in the same manner as in Document 2, the functions of the catalyst layer are allocated only in the thickness direction of the catalyst layer. It is thus difficult to secure a three-phase interface where electrode reaction takes place, and the power generating characteristics at high current densities degrade.
The invention solves these problems associated with conventional art, and intends to provide a direct oxidation fuel cell that is excellent in power generating characteristics and durability.